God is the Judge We are the Messengers

It is not the believers business to decide who is saved and who is not in the church. However it is our business to point out sin in the body when we see it. We might even challenge someone to ask God if they are saved because there doesn't seem to be any shame over sin.

Hebrews 12:5-7
"…5 And you have forgotten the exhortation that addresses you as sons: “My son, do not make light of the Lord’s discipline, or lose heart when He rebukes you. 6 For the Lord disciplines the one He loves, and He chastises everyone He receives as a son.” 7 Endure suffering as discipline; God is treating you as sons. For what son is not disciplined by his father?…"

In Matthew 7:21-23 God warns us that many will think they are saved, only to be turned away when they get to the judgement.

Matthew 7:21-23 "21 “Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but the one who does the will of my Father who is in heaven. 22 On that day many will say to me, ‘Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many mighty works in your name?’ 23 And then will I declare to them, ‘I never knew you; depart from me, you workers of lawlessness.’"

We are not to decide who is saved and who is not when they claim to be believers, but we can boldly declare their sin that has already been judged by God.